1. Field of the Invention
The present invention relates generally to image processing apparatus and methods and particularly to image processing apparatus and methods employing an error distribution method to perform a halftone process intensively providing dots to achieve both high resolution and a high level of tone representation.
2. Description of the Related Art
In electronic photography, dithering intensively providing dots is often employed to stabilize dots that are output. Dithering, however, has a disadvantage that it hardly provides a high level of tone representation and high resolution simultaneously. More specifically, if high resolution is desired and a dithering pattern is accordingly reduced then tone representation is impaired and if a high level of tone representation is desired and a dithering pattern is accordingly increased then the high level of resolution would not be achieved.
For example if a printer having a resolution of approximately 600 dpi, a currently main level of resolution, is used to achieve a high resolution of 150 lines, a dithering pattern of 4×4 would be employed. This only provides tone representation of 4×4 or 16 gray levels and natural image representation would not be achieved, since it requires at least approximately 60 gray levels. If a dithering pattern of approximately 8×8 is employed to achieve a tone representation of approximately 60 gray levels, however, resolution would be decreased to 75 lines.
Reiner Eschbach suggests intensively providing dots by adding a large dithering pattern to a threshold value in an error distribution method to achieve both a high level of tone representation and high resolution in a halftoning process. To intensively provide dots in this technique, a signal (i.e., a dithering pattern) approximately five times in magnitude the range of an input must be added to the threshold value. In this technique, a large pattern signal is added and the resultant image is thus impaired in sharpness. As such an edge emphasis process is also required. The technique is also disadvantageous in that the threshold can hardly have added thereto a pattern formed of closely arranged parallel lines. While this technique does establish both a high level of tone representation and a high resolution, it does not provide sufficiently fine images.
Thus an image processing including dithering is disadvantageous as it fails to provide high resolution and a high level of tone representation simultaneously. Reiner Eschback's error distribution method is also disadvantageous as (1) it does not provide an image with better granularity than dithering, (2) it can hardly employ a pattern formed of closely arranged parallel lines, (3) to output for example a black letter clearly, an edge emphasis process (for example providing a threshold value in inverse proportion to an input) is required, and (4) an input significantly changes a texture and a pseudo contour thus results.